The present invention is directed to electronic packaging, and more particularly, to Land Grid Array interconnections.
The term Land Grid Array (LGA) refers to a design option used by device manufacturers to package their devices without any terminations, such as solder balls, on the bottom of the chip carrier substrate. In order for LGA devices to electrically connect to a circuit board there is an intermediate device with contacts known as “fuzz buttons”. Fuzz buttons are commonly manufactured from a single strand of wire and compressed into a cylindrical shaped bundle. The fuzz buttons are inserted between the contact pads on the bottom of the chip carrier substrate and their corresponding contact pads on the circuit board. When the chip carrier is forced against the circuit board, the fuzz-buttons compress between the respective contact pads to form the electrical connection.
Land grid array connections are typically used in high-end applications such as the substrate-to-board attach of high input-output (I/O) count packages. A high I/O count package will contain chip carriers with many contact pads. As RF frequencies, current densities, and I/O counts increase, while chip carrier substrate size and pitch continue to decrease, LGA connections are becoming more commonly used across a broad range of applications.
The fuzz button contact is made by randomly winding wire, typically gold plated molybdenum or tungsten, into a cylindrical bundle or “button”. The fuzz buttons are typically loaded into a custom molded insulator or interposer configured to the geometry and pitch of the desired application. In other words, the fuzz buttons will coincide with the chip carrier and circuit board contact pads. In this way the fuzz buttons provide the electrical contact between chip carrier and circuit board. This configuration is commonly referred to as an “LGA socket”.
Two major difficulties in manufacturing the current LGA modules are achieving the proper chip carrier substrate contact pad coplanarity (flatness) and controlling the variations with fuzz button contact characteristics such as height and textures of wire webbing. These characteristics are essential to ensure that adequate electrical contacts are made and reliably maintained. The current LGA contact pad is basically flat with no topography. The disadvantages of flat contact pads are the requirement of an excellent chip carrier substrate coplanarity or flatness which results in production yield loss.
Another problem with LGA modules is the requirement of a significant external actuation force to achieve a good electrical contact between chip carrier substrate contact pads, fuzz buttons and circuit board contact pads for the second level packaging. This high external actuation force may cause chip carrier substrate cracks and stress induced corrosion cracks. Another problem with LGA modules is the need for additional fixtures to enhance the electrical contact and prevent the loss of alignment or “walking” of the fuzz buttons and interposer due to the lack of mechanical inter-locking between contact pads and fuzz buttons.
The present invention is to use micro-bumps with controlled geometry, topography and distributions on the contact pad to resolve the above problems. The manufacturing process of producing the disclosed micro-bumps on the contact pads is also disclosed. More particularly, the present invention provides a controlled surface topography on Input-Output (I/O) metal pads on the Land Grid Array (LGA) of microelectronic packages. The result is to enhance the surface areas of contact between the I/O pads in LGA substrates and the fuzz buttons in the interposer.
Thus, a purpose of the present invention is to build micro-bumps on substrate I/O contact pads to improve the physical LGA contact area.
Another purpose of the present invention is to provide mechanical inter-locking between the fuzz-button and contact pad and thereby reduce the displacement of electrical contact points during thermal excursions.
Another purpose of the present invention is to reduce the required actuation force.
These and other purposes of the present invention will become more apparent after referring to the following description considered in conjunction with the accompanying drawings.